1. Field of the Invention
Embodiments of the invention are generally directed to the field of acoustic transducers. More particularly, embodiments of the invention are directed to a hybrid geometry piezoelectric transducer and methods for making such a transducer.
2. Description of Related Art
Electromechanical transducers are used for the interconversion of electrical and mechanical energy. In acoustic applications, these include, but are not limited to, microphones, speakers, underwater projectors, hydrophones, sonar, sonic cleaning and imaging, and weaponry. In a typical solid-state transducer, the acoustically active element is made from a piezoelectric ceramic material such as lead zirconate titanate (PZT), an electrostrictive ceramic such as lead magnesium niobate (PMN), a magnetostrictive metal alloy such as Terfenol-D, or other similar active material. (See, e.g., B. Jaffe et al., Piezoelectric Ceramics, Academic Press, London, N.Y., 1971 and http://www.etrema.com/core/terfenold).
Two common, albeit dissimilar, types of acoustic transducers are resonant transducers such as the Tonpilz electroacoustic transducer and non-resonant, bulk-mode PZT composite transducers.
The basic configuration of the modern Tonpilz transducer is disclosed in Massa U.S. Pat. No. 3,328,751 and is illustrated in FIG. 1. The Tonpilz resonator includes a stack of a piezoceramic (PZT) material (e.g. annular pieces, rings) bolted together in a prestressed condition between a relatively massive tailmass and a relatively lighter, flared headmass. For a more in-depth description, the interested reader is directed to, e.g., O. B. Wilson, Introduction to Theory and Design of Sonar Transducers, ch. 6, Peninsula Publishing, Los Altos, Calif., 1991, the disclosure of which is incorporated herein by reference in its entirety to the fullest allowed extent.
PZT composite type transducers, typically referred to as 1-3 composites or 2-2 composites, are geometrically configured differently than the Tonpilz type resonator. The 1-3 composite transducer has a one-dimensionally connected ceramic phase (e.g., PZT columns or pillars) contained within a three-dimensionally-connected matrix provided by an organic polymer phase. A schematic illustration of a basic transducer of this type is shown in FIG. 2. The 2-2 composite transducer comprises two-dimensionally-connected strips of PZT ceramic separated by two-dimensionally-connected parallel strips of polymer. This configuration is widely used in phased array type ultrasound transducers. For more detail regarding the implementation and process of manufacture of composite transducers, the interested reader is directed to, e.g., U.S. Pat. Nos. 4,728,845; 5,334,903; and 5,340,510, the disclosures of which are incorporated herein by reference in their entireties to the fullest allowed extent.
Another type of transducer is a hybrid material transducer in which active electrostrictive and magnetostrictive transducer materials are intimately combined in a unitary transducer construction. In this type of hybrid transducer, the mixture of two dissimilar active materials provides electrical and mechanical advantages not available to a single transducer type. The interested reader is directed to, e.g., Butler et al., U.S. Pat. No. 4,443,731, the disclosure of which is incorporated herein by reference in its entirety to the fullest allowed extent.
Each of these different types of transducers have various advantages and disadvantages depending upon their applications, as well as limitations and tradeoffs that affect their ultimate performance. Special considerations are directed to transducer size, weight, strength, environmental and operational durability, operating frequency, sensitivity, noise performance, radiation response and directionality, baffling, cost, and other physical, structural and performance related attributes well recognized by persons skilled in the art.
More particularly, for example, resonant transducers like the Tonpilz type can provide high sensitivity and high output, but typically at a reduced bandwidth and not constant across frequency. In addition, the wide variation of electrical impedance near resonance can pose design challenges and require certain compromises. Furthermore, a Tonpilz resonator requires a bulky headmass that is subject to unwanted deformation and other known issues.
Traditional composite transducers use elastomeric filler between ceramic rods or posts. Although the elastomeric filler provides increased strength, transducers built from composites (such as 1-3 ceramic) tend to suffer from reduced sensitivity as well as vibro-acoustic crosstalk due to the composite filler material. For a transducer of a given size, the presence of filler material decreases the effective compliance and lowers output and sensitivity. Methods to counteract this problem have been demonstrated. Some methods have included using gas-voided polymers as a fill material to reduce the shear wave velocity and increase compliance. Another example is the negative Poisson ratio polymers proposed by Smith (e.g., U.S. Pat. No. 5,334,903); however, both these methods are inherently narrowband, temperature dependent, and can exacerbate the problems caused by lateral resonances. Lateral resonances could be broadly defined as undesirable or deleterious vibrational or acoustical propagation normal to the preferred transducing direction. These resonances can occur from the presence of the filler and consequently generate non-uniformity in the frequency response. Through general reciprocity and the act of transduction, the non-uniformities propagate to all inputs and outputs of the device, appearing in the mechanical, electrical, and acoustical frequency responses. Along the same lines, fillers tend to make sensors constructed in this manner sensitive to sound coming from the wrong direction. Various methods to block orthogonal signals using absorptive materials or baffles can be employed, which may incur system cost and weight penalties. Moreover, composite fillers are generally made from elastomers like rubber or polyurethane, whose properties are usually highly dependent on temperature. Changes in temperature may result in unacceptable changes in key performance attributes resulting from changes in material compliance, sound speed, and other properties. Another disadvantage of transducers fabricated from composites (as well as those built using hollow ceramic cylinder or sphere configurations) is that they often are necessarily made structurally weak in order to obtain high sensitivity. FIGS. 3(a-d) show comparative graphs of hydrophone receive sensitivity of several different transducer configurations. With a similar volume of active material, a Tonpilz-style device shows some clear advantages in sensitivity. Designs with omni-directional (or multipole-like) radiation responses can less effectively discriminate against noise and other interferers without using several transducers in an array configuration. Typically, spherical designs produce omni-directional responses and composites have a dipole or quadrupole-like response that corresponds to the primary and secondary axis in the material. As a consequence, they incorporate bulky acoustic baffling or may use multiple sensors to achieve a directional response. FIGS. 4(a-d) show comparative sensing footprints of several different types of transducers.
In view of all of the foregoing considerations and others that are appreciated by persons skilled in the art, the inventor has recognized a need for an acoustic transducer design, construction, and method for making that address the known shortcomings of conventional transducers and provide improvements over the various attributes of the conventional transducer types mentioned above.